Field of Invention
The present invention relates to a calibrating circuit. More particularly, the present invention relates to a calibrating circuit and a calibrating method for a display panel.
Description of Related Art
FIG. 1 is a circuit diagram illustrating an active matrix organic light emitting diode (AMOLED) display in the prior art. The AMOLED display includes a gate driver 110, a source driver 120 and a display panel 130. The display panel 130 includes scan lines (e.g. a scan line S_1 and a scan line S_2), data lines (e.g. a data line D_1 and a data line D_2) and pixel circuits (e.g. a pixel circuit 131). The pixel circuit 131 includes a switch 132, a driving transistor 133 and an organic LED (OLED) 134.
The gate driver 110 sequentially scans scan lines of the display panel 130, so that the source driver 120 can write data voltages into the pixel circuits. Take the pixel circuit 131 as an example, during a period that the gate driver 110 turns on the switch 132 through the scan line S_1, the source driver 120 transmits a data voltage to the gate of the driving transistor 133 through the data line D_1 and the switch 132. The gate voltage of the driving transistor 133 determines a current I1 of the driving transistor 133. The current I1 flowing through the OLED 134 determines brightness of the OLED 134. The relationship formula between the gate-source voltage of the driving transistor 133 and the current I1 is written as I1=k(VGS−Vt)2, where k denotes a real number, VGS denotes the gate-source voltage of the driving transistor 133, and Vt denotes the threshold voltage of the driving transistor 133. Different driving transistors may have different threshold voltages because a process drift or other factors. The difference between the threshold voltages may cause mura (i.e. uneven brightness) or other defects. If the threshold voltage of the driving transistor 133 is sensed, then the source driver 120 can adjust the data voltage written into pixel circuit 131 to compensate the drift of the threshold voltage.
In general, a compensation circuit is disposed outside the display panel 130 to sense the threshold voltage of the driving transistor 133. The sensed threshold voltage is transmitted through an amplifier and an analog to digital converter (ADC) to obtain a digital code which is used to calibrate the threshold voltage. However, different amplifiers may have different gains due to a process drift or other factors.